A typical membrane keyboard of the prior art includes three membranes or layers of flexible sheet material. Electrical conductors are screened onto the bottom surface of the top layer and the top surface of the bottom layer. The center layer is provided with a plurality of holes including a hole at each switch site. The arrangement is such that when the top layer is depressed a conductor on its lower side makes contact with a conductor on the upper side of the bottom layer through a hole in the center layer.
Keyboards as described above may have an adhesive which seals the top and bottom layers to the center layer. This arrangement is not suitable for use in very low force (i.e. 5-15 grams) keyboards because pressure builds up in the sealed switch region as actuating force is applied to the top membrane. To overcome this problem some membrane keyboards have been constructed without the adhesive. The layers are not glued but are held together mechanically in regions remote from the switch sites. However, membrane keyboards of this type exhibit a hysteresis in that the displacement of the top layer in response to a force applied thereto is not the same when the top layer is depressed as when it is released. As the top layer is depressed, the pressure increases in the switch region bounded by the top and bottom layers and the edge of the hole in the center layer. This increased pressure tends to separate the top and bottom layers from the center layer because they are not glued together or mechanically held together in the switch region, and the air is dispersed into the regions between the layers. However, as the force on the top layer is released, the pressure in the switch region drops and the air begins to flow back between the layers to the switch region. This flow causes a reduced pressure between the layers which draws the bottom and top layers against the center layer in the region around the hole in the center layer. A vacuum is thus created in the switch region that retards the return of the top layer as the force on the top layer is released. This prevents a quick clean break of the contacts and may result in intermittent opening and closing of the contacts thus producing false signal levels. This is particularly true when the actuator for applying force to the top layer includes a buckling spring or rubber dome spring. Any bouncing of the spring causes a bouncing movement of the top layer. If the top layer is retarded by the vacuum in the switch region, it will be closer to the bottom layer during the bouncing and thus more likely to again make contact after the initial contact is broken. A similar problem exists in keyboard arrangements wherein the keystem acts directly against the top membrane. In this case rocking of the keystem by the operator, rather than spring bounce, may cause further contact after the initial contact is broken.
The prior art provides many solutions to the problem. In one approach, vent passages are formed in the upper or lower layers to permit free air flow between each switch region and the exterior of the keyboard. This solution has a disadvantage in that additional manufacturing steps are required to form vent passages in one or more of the layers. In a second approach, a maze of passages is formed in the center layer and interconnects the switch regions. The maze may be sealed off from the exterior environment on the theory that the volume change at one switch site as a result of pressing the top layer is insignificantly small compared to the total volume of all switch regions and the interconnecting passages so that the pressure in a switch region remains substantially constant. This arrangement requires a center layer which is hard to handle during assembly because of the many passage cut-outs. Also, since the assembly is sealed, the force required to close the contact increases or decreases as the pressure in the surrounding environment decreases or increases. In an extreme case, a high environmental pressure may cause switch actuation without any force being applied by an operator.
U.S. Pat. No. 4,317,013 solves the problem of pressure imbalance by dispensing with the center layer. Spacer areas are screened in a uniform pattern onto either the top or bottom layer, or both, and the top and bottom layers are glued together by glue applied to the spacer areas. A grid-like series of passages thus separates the top and bottom layers over their entire surfaces. This arrangement had the disadvantage that the spacer areas must be applied in a separate operation subsequent to the screening of the conductors onto the top and bottom layers because the spacers are also located over the switch contacts. Also, a further assembly step is required to apply the glue.
U.S. Pat. No. 4,391,845 also dispenses with the center layer and employs spacer areas which are screened onto the top or bottom layer. During a first pass the conductors and spacers are simultaneously screened onto a layer. Subsequent screening passes are then required to build up the thickness of the spacers because the thickness of the spacers after one screening pass is insufficient to reliably maintain a spacing between the conductors on the top and bottom layers.